How does it work? Do users provide a buffer and the kernel fills the buffer with data and notifies the user when ready?

That is more akin to AIO Linux system, then? Otherwise, epoll/poll/select just notifies users when data is available but the actual copy is done by the user. Surprisingly this can make a huge difference when streaming large amounts of data.

http://blog.lighttpd.net/articles/2006/11/12/lighty-1-5-0-an...

We have argued here before and I have gotten downvoted into oblivion for being pedantic and distinguishing between asynchronous IO and non-blocking IO but it looks like that extra user-space memcpy can make a huge difference.

I can't find anything about this now, just spent a good 20 minutes searching for it. I guess keywords kqueue, buffer request http are too generic in some sense. :-/

Anyway, the idea was to avoid context switches by waiting/parsing in kernel-side till there was enough data for the client to do something else that just another gimme_more_data()-call back to the kernel.

It could even be applied to other methods than kqueue, so perhaps I remember a bit wrong that this was just for kqueues.

That's actually accept_filter(9). The man page for freebsd has a interesting info:

    The accept filter concept was pioneered by David Filo at Yahoo! and
    refined to be a loadable module system by Alfred Perlstein.

The closest you can get by using kqueue is to set a low water mark, so that a read event is only returned when there's enough data ready.

Ah, that's it! Thanks for finding it.